This invention relates to a control for a hydraulic excavator and in particular to an excavator control of the type that is used in conjunction with a laser generator capable of generating a laser beam or plane.
The use of lasers to automatically control earth moving equipment is known in the construction industry. Some limited success has been achieved in controlling bulldozer blade elevations as well as controlling the blade elevation of motor graders. One piece of construction equipment that has eluded successful application of a laser is the excavator. A desirable attribute for an excavator would be one that could easily and quickly dig exactly to a finished grade at a desired depth without the requirement for frequent operator checks or for requiring additional workers in the area. The system should be easy to operate and function properly even though the excavator changes its elevation and attitude frequently. In addition, an excavator should allow the operator to dig to a level grade or to a nonlevel grade having a desired slope or percent of grade.
Several attempts have been made at such an excavator control but all have had serious disadvantages. For example, in U.S. Pat. No. 4,129,224 issued to Ted L. Teach for an AUTOMATIC CONTROL OF BACKHOE DIGGING DEPTH, a laser beam sensor unit is mounted on the end of a pendular mast pivotally mounted by the boom pivot pin. A vertical motor continuously adjusts the vertical height of the mast to keep the laser beam sensor in the plane of the laser beam. A transducer monitors the amount of extension of the mast and produces an electrical signal proportional to the height of the mast and hence proportional to the absolute vertical spacing between the pivot axis of the boom and the laser plane. Angular displacement transducers monitor the angles between the backhoe frame and the boom, between the boom and the stick and between the stick and the bucket. The position of the bucket cutting teeth with respect to the backhoe can be determined as a trigonometric relationship between the three angles. By combining the distance from the laser receiver to the backhoe and from the backhoe to the cutting edge the true depth of the cut should be determinable.
Such a device has several drawbacks. The laser height-seeking detector requires a mast, that not only extends above the excavator and is therefore vulnerable to damage, but also requires means such as pendular mounting to maintain the mast vertically aligned. In a conventional excavator, the boom pivot is typically disposed under the cab or other obstruction, so application of a mast becomes impractical. In addition, a beam-seeking drive motor and transducer are required. Further, while the angular displacement transducers may lend themselves well to implementation in a control using analog circuits, such circuits may be ambient temperature sensitive and the trigonometric relationships between the cutting edge of the bucket and the transducer outputs makes direct conversion to digital control prohibitive. The reason this is prohibitive is that a digital computer traditionally performs trigonometric calculations by successive approximation, an iterative trial and error process. While such calculations pose no problem in the laboratory setting, they are much too slow for real-time control of a dynamic machine such as an excavator.
In U.S. Pat. No. 4,231,700 issued to Robert H. Studebaker for a METHOD AND APPARATUS FOR LASER BEAM CONTROL OF BACKHOE DIGGING DEPTH, the laser receiver is mounted to the stick member rather than on the end of a pendular mast. However, just as with Teach '224, the laser receiver must at all times stay in contact with the laser beam to function. Studebaker suggests that by directing a laser beam along a plane that is a predetermined elevation with respect to the desired dig depth, a manual or automatic control could be made to cause the cutting edge of the bucket to dig to a predetermined depth. While some of the difficulties of the previous devices are overcome, the limitations of this device are apparent. The location of the laser plane is extremely inflexible. If even a moderately-deep ditch is being dug, the laser plane must be located below ground level. Further, the other difficulties inherent in Teach '224 are not even addressed in Studebaker.
Other solutions have been suggested, but they all require that the position of the excavator be located with a transit and relocated every time the frame moves. This not only is a time-consuming task but the precise position of the frame is subject to frequent and often abrupt changes during the operation of the excavator. As a result, digging progress is not only slow but is also likely to be inaccurate.